[Doc] Add flow as func doc (#1353)

This pull request adds a new guide to the documentation on how to
execute a flow as a function. The guide explains how to load and invoke
a flow as a function in code, configure the flow with context, override
flow inputs, and use streaming output.

![image](https://github.com/microsoft/promptflow/assets/7776147/aebfa891-3b70-4eb5-861d-0189dbacc8a4)

* <a
href="diffhunk://#diff-70d4090f17d29e6309aa88a93da052dbee39016c0c63956049f11814d7afc4b3R19">`docs/how-to-guides/index.md`</a>:
Added a new guide on how to execute a flow as a function.
* <a
href="diffhunk://#diff-b01a987e6744c2de5b19ecd68893c154271b0350749ad6512dc7de20981e7a4fR1-R63">`docs/how-to-guides/execute-flow-as-a-function.md`</a>:
Added a new guide explaining how to load and invoke a flow as a function
in code, configure the flow with context, override flow inputs, and use
streaming output.# Description

Please add an informative description that covers that changes made by
the pull request and link all relevant issues.

# All Promptflow Contribution checklist:
- [ ] **The pull request does not introduce [breaking changes].**
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- [ ] **Create an issue and link to the pull request to get dedicated
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- [ ] There are a small number of commits, each of which have an
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### Testing Guidelines
- [ ] Pull request includes test coverage for the included changes.

docs/how-to-guides/execute-flow-as-a-function.md

@@ -0,0 +1,92 @@
+# Execute flow as a function
+
+:::{admonition} Experimental feature
+This is an experimental feature, and may change at any time. Learn [more](faq.md#stable-vs-experimental).
+:::
+
+## Overview
+
+Promptflow allows you to load a flow and use it as a function in your code.
+This feature is useful when building a service on top of a flow, reference [here](https://github.com/microsoft/promptflow/tree/main/examples/tutorials/flow-deploy/create-service-with-flow) for a simple example service with flow function consumption.
+
+## Load an invoke the flow function
+
+To use the flow-as-function feature, you first need to load a flow using the `load_flow` function.
+Then you can consume the flow object like a function by providing key-value arguments for it.
+
+```python
+f = load_flow("../../examples/flows/standard/web-classification/")
+f(url="sample_url")
+```
+
+## Config the flow with context
+
+You can overwrite some flow configs before flow function execution by setting `flow.context`.
+
+### Load flow as a function with in-memory connection override
+
+By providing a connection object to flow context, flow won't need to get connection in execution time, which can save time when for cases where flow function need to be called multiple times.
+
+```python
+from promptflow.entities import AzureOpenAIConnection
+
+connection_obj = AzureOpenAIConnection(
+    name=conn_name,
+    api_key=api_key,
+    api_base=api_base,
+    api_type="azure",
+    api_version=api_version,
+)
+# no need to create the connection object.  
+f.context = FlowContext(
+    connections={"classify_with_llm": {"connection": connection_obj}}
+)
+```
+
+### Local flow as a function with flow inputs override
+
+By providing overrides, the original flow dag will be updated in execution time.
+
+```python
+f.context = FlowContext(
+    # node "fetch_text_content_from_url" will take inputs from the following command instead of from flow input
+    overrides={"nodes.fetch_text_content_from_url.inputs.url": sample_url},
+)
+```
+
+**Note**, the `overrides` are only doing YAML content replacement on original `flow.dag.yaml`.
+If the `flow.dag.yaml` become invalid after `overrides`, validation error will be raised when executing.
+
+### Load flow as a function with streaming output
+
+After set `streaming` in flow context, the flow function will return an iterator to stream the output.
+
+```python
+f = load_flow(source="../../examples/flows/chat/basic-chat/")
+f.context.streaming = True
+result = f(
+    chat_history=[
+        {
+            "inputs": {"chat_input": "Hi"},
+            "outputs": {"chat_output": "Hello! How can I assist you today?"},
+        }
+    ],
+    question="How are you?",
+)
+
+
+answer = ""
+# the result will be a generator, iterate it to get the result
+for r in result["answer"]:
+    answer += r
+
+```
+
+Reference our [sample](https://github.com/microsoft/promptflow/blob/main/examples/tutorials/get-started/flow-as-function.ipynb) for usage.
+
+## Next steps
+
+Learn more about:
+
+- [Flow as a function sample](https://github.com/microsoft/promptflow/blob/main/examples/tutorials/get-started/flow-as-function.ipynb)
+- [Deploy a flow](./deploy-a-flow/index.md)

docs/how-to-guides/index.md

@@ -10,6 +10,7 @@ init-and-test-a-flow
 add-conditional-control-to-a-flow
 run-and-evaluate-a-flow/index
 tune-prompts-with-variants
+execute-flow-as-a-function
 deploy-a-flow/index
 enable-streaming-mode
 manage-connections